Polycystic ovary syndrome -- the most common cause of female infertility in the United States -- causes higher than normal levels of the male hormone androgen, infrequent periods and small cysts on the ovaries. It affects 5 to 10 percent of reproductive-age women, according to background information in the study.

"We found a simple and comparatively safe and vastly more effective treatment for [polycystic ovary syndrome]," said lead researcher Dr. Richard Legro, a professor of obstetrics and gynecology at Penn State University's College of Medicine in Hershey, Penn.

Clomiphine, which works by stimulating ovulation, has been the standard treatment for years, but has a high rate of multiple births, Legro said.

Letrozole, a treatment for breast cancer in postmenopausal women, works by blocking estrogen production, tricking the ovaries into producing more of the hormone, he explained.

The new study, funded by the U.S. National Institutes of Health, was published July 10 in the New England Journal of Medicine.

Researchers found that almost 28 percent of the women taking letrozole had babies after five cycles, compared with about 19 percent of those taking clomiphene.

"Clomiphine may be trumped," Legro said. "To see a 40 percent improvement in birth rate is a huge difference."

Whether letrozole will cause more birth defects than other drugs isn't known, Legro said. "We will need other studies to show that the rates of birth defects are actually low -- lower than we would expect in an infertile population," he said.

Letrozole, a generic drug, is relatively inexpensive. Thirty pills cost around $56, similar to the cost of clomiphene, Legro said. "This is an affordable form of infertility treatment," he said.